drm/i915/gvt: Fix crash after request->hw_context change

When we do shadowing, workload's request might not be allocated yet,
so we still require shadow context's object. And when complete workload,
delay to zero workload's request pointer after used for update guest context.

v2: Move request alloc earlier as already try to track shadow status
depending on request state, which also facilitate to use request->hw_context
for target engine context reference.

Fixes: 1fc44d9b

 ("drm/i915: Store a pointer to intel_context in i915_request")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Cc: Weinan Li <weinan.z.li@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180521081752.31056-1-zhenyuw@linux.intel.com